This invention relates to a method for fabricating an electrode, particularly a stimulating electrode, implantable in an organism such as the human body.
Stimulating electrodes, for example, for heart pacers generally comprise an insulated cable lead and an electrode in the form of a hemisphere, a cylinder or a wire, for transmitting the stimulation pulses. Stimulation of the heart by electrical pulses, when the propagation of the stimulation is interrupted, consists of the generation of a given electric field strength at an excitable cell membrane. After the stimulation or activation is triggered, it propagates independently over the entire heart muscle and leads to its contraction.
The electrical stimulation pulses are generated by an electronic pacer which consists of an implantable electronics parts having a power supply unit and a stimulation circuit including a stimulating electrode and an indifferent electrode. During the stimulation pulse, a small capacitor is partially discharged through the stimulation circuit within 0.5 to 2 milliseconds. In the pauses between consecutive pulses the capacitor is recharged from the power supply unit, i.e., a battery. During the pulse, the field strength required for triggering the contraction of the heart muscle is present in the stimulatable tissue in the vicinity of the stimulating electrode.
Conventional stimulating electrodes, for example, of platinum, platinum-iridium or an alloy of 40 parts cobalt, 20 parts chromium, 16 parts iron, 15 parts nickel, 7 parts molybdenum and 2 parts manganese (Elgiloy) cause degeneration of the adjoining tissue because they surround themselves within a period of two to four weeks with a connective tissue layer about 0.5 to 1 mm thick which is not stimulatable. During the development of the connective tissue layer, the stimulation threshold increases steadily, i.e., an increasingly larger current is required for triggering heart contractions. The required voltage also increases. Because the distance between the stimulatable tissue and the electrode increases, more energy must be supplied to generate the same field strength at the stimulatable tissue. If the head of the stimulating electrode consists, for example, of a hemisphere with a radius of 1 mm and a connective tissue layer approximately 1 mm thick forms around the electrode, the stimulation threshold current increases fourfold. Since the voltage increases approximately to the same extent, the power required increases approximately 16 times. It is, therefore, clear that requirements as to capacity and voltage of the energy source depend to a considerable degree on the growth of the tissue at the stimulating electrode.
As described by Z. Marinkovic and R. Roy in "Preparation and Properties of Sputtered `Glassy` Carbon Films," Carbon, 1976, Vol. 14, pages 329-31, vitreous carbon can be sputtered onto glass substrates, the carbon attaining a density of 0.5 to 1.79 g/cm.sup.3. The deposition rate of vitreous carbon increases proportionally with the power, but the density of this sputtered-on vitreous carbon layer is inversely proportional to the power. Under the same conditions for sputtering carbon, a density of 1.58 g/cm.sup.3 may be obtained for graphite as the target material, a density of 1.40 g/cm.sup.3 for pyrolytic graphite, and a density of 0.74 g/cm.sup.3 for vitreous carbon. The layer deposited by sputtering is not crystalline and its microstructure is grainy; it contains grains on the order of magnitude of 0.1 to 1.0 .mu.m.
It is known to use vitreous carbon (also called "glassy carbon") as the electrode material for stimulating electrodes. An electrode head made of vitreous carbon material is superficially activated, i.e., has a surface with microporous structure in which the diameter of the pores is smaller than 0.002 .mu.m. By activating the vitreous carbon of the electrode head, the polarization losses which occur at the boundary surface between the electrode and the tissue and which do not contribute to an increase of the field strength in the adjoining stimulatable tissue can be kept very low. An implantable electrode is thereby obtained which ensures little encapsulation by connective tissue, as well as low energy consumption and, concomitant therewith, good steady-state operation. The reason for these advantages is that the current density of the stimulation threshold does not increase for the duration of the implantation. In this known electrode the electrode head is activated in a separate operation by heating it in air to temperatures about 400.degree. C. A slight burn-off occurs at the surface, which has a beneficial effect on the electrical properties. Macroscopically, the smooth surface of the electrode head is preserved. (See German Auslegeschrift No. 26 13 072.)
An object of the invention to provide a simple method of manufacturing an implantable electrode, especially a stimulating electrode, the electrode material of which is compatible with the body and has a large specific electrochemical double-layer capacity relative to the area of the electrode.